FIG. 1 schematically illustrates the structure and the function of a magnetic tunnel junction bearing the reference 1. In a known manner, the magnetic tunnel junction (or magnetoresistive stack) 1 is composed of two magnetic layers 2 and 3 separated by a layer of oxide 4 forming a tunnel barrier, typically made of aluminium or magnesium oxide. The magnetisation of one of the magnetic layers 2, known as storage layer (layer with magnetisation re-orientable along two substantially opposite directions) may be oriented in different directions with respect to the magnetisation of the second layer 3, known as reference layer, the magnetisation of which is pinned in a fixed direction. This pinning is in general achieved by interaction with an adjacent antiferromagnetic layer not represented (exchange anisotropy mechanism). Different levels of resistance of the tunnel junction may be produced as a function of the angle between the magnetisations of the two storage and reference layers. Thus the information is stored in the magnetic element by the parallel or antiparallel magnetic configuration of the magnetisations of the storage 2 and reference 3 layers. Then the variation in resistance as a function of the magnetic configuration is used to reread the information written in the memory cell. When the magnetisations of the magnetic layers 2 and 3 are antiparallel, the resistance of the junction 1 is high; when the magnetisations are parallel, the resistance becomes low. The variation in resistance between these two states may exceed 100% by an appropriate choice of materials. Each tunnel junction 1 constitutes a memory point.
In the most conventional approach of MRAMs with writing by perpendicular magnetic fields, the tunnel junctions 1 are arranged in a square array inserted between two perpendicular arrays of parallel lines: bit lines 5 and word lines 6, one above, the other below the plane of the tunnel junctions 1. The junctions 1 are placed between a transistor 7 and a bit line 5. A current passing in this bit line 5 produces a magnetic field. A current passing in the word line 6 orthogonal to the bit line 5 makes it possible to produce a second magnetic field. At the moment of writing, the transistor 7 is blocked and current pulses are sent simultaneously into the word line 6 and the bit line 5 which intersect at the level of the addressed memory point 1. The combination of these two fields makes it possible to switch the magnetisation of the storage layer of the addressed memory point 1 in the desired direction without affecting the magnetisation of the other memory points. In “reading” mode, the transistor 7 is in saturated regime. The current sent into the bit line 5 uniquely passes through the memory point of which the transistor is open. This current makes it possible to measure the resistance of the junction. By comparison with a reference memory point, the binary state of the memory point (“0” or “1”) may thus be determined.
More recently other types of magnetic devices have appeared in which the reversal of magnetisation takes place no longer by external magnetic fields but by using the action exerted by a spin polarised current which enters the storage layer from the tunnel junction. When a spin polarised current is injected into a magnetic nanostructure, this current exerts a torque on the magnetisation of the nanostructure, known as “spin transfer torque” or “spin-torque”, which can make it possible to act on the magnetisation of the nanostructure and in particular to re-orientate it in a desired direction. Such magnetic devices are designated by the terminology STT-MRAM for “Spin-Transfer Torque MRAM”).
In order to make switching of STT-MRAM more rapid while overcoming phenomena of stochastic switching, a solution proposed in the patent FR2817998 consists, to cause the switching of a magnetic layer of planar magnetisation, in injecting into this layer a spin polarised current, the direction of polarisation of which is perpendicular to the plane of the layers. Such a magnetic device 30 (designated by the terminology OST-MRAM for “Orthogonal Spin-Transfer MRAM”) is illustrated in FIG. 2. As for the device of FIG. 1, the device 30 represented includes two magnetic layers 12 and 16 separated by a spacer 14 forming a tunnel barrier, typically made of aluminium or magnesium oxide. The magnetisation of the magnetic storage layer 16 may be oriented along two substantially opposite directions with respect to the magnetisation of the reference layer 12, the magnetisation of which is pinned in a fixed direction. The storage layer 16 may be a simple ferromagnetic layer or a synthetic antiferromagnetic layer constituted of two ferromagnetic layers coupled in an antiparallel manner through a thin antiparallel coupling layer. The same is true for the reference layer 16.
The assembly 12, 14, 16 constitutes a magnetic tunnel junction 15. The device 30 is completed by a separating non-magnetic conducting layer 18 and a magnetic polarisation layer 20 (also designated hereafter by the term perpendicular polariser) having a magnetisation perpendicular to the plane of the layer. The perpendicular polariser 20 may be a simple layer or multilayer or a synthetic antiferromagnetic layer/multilayer as described for example in the article “Improved coherence of ultrafast spin-transfer-driven precessional switching with synthetic antiferromagnet perpendicular polarizer” (Vaysset et al., Applied Physics Letters 98 (2011) 242511).
The polarisation layer lies on a conducting electrode 22. The whole of this stack is inserted between a current distribution not represented and a current switching transistor 26. For the electrons transmitted (or reflected) through the layer 20, the spin direction will be oriented parallel (or antiparallel) to the magnetisation of this layer, that is to say perpendicularly to the plane of the various layers of the junction 15 and in particular to the plane of the storage layer 16. The magnetisation of this layer subjected to this out of plane polarised current of electrons is going to turn along a cone of large angle of axis Oz perpendicular to the plane of the layer, without being able to align itself with the direction of spin due to the demagnetising field which tends to maintain the magnetisation in the plane of the layer. It will be noted that, in this situation where the storage layer 16 has planar magnetisation, the junction preferentially has an elliptical shape assuring a shape anisotropy in the plane favouring a stable alignment of the magnetisation of the storage layer along the large axis of the ellipse.
FIG. 3 symbolically illustrates this rotation. In this device, the spin transfer generated by the current polarised by the perpendicular polariser 20 induces a large angle precessional movement of the magnetisation of the storage layer 16. This precessional movement arises from the fact that the magnetisation of the storage layer 16 tends to go slightly out of plane under the effect of the spin transfer stemming from the perpendicular polariser 20 and then to turn in the out of plane oriented demagnetising field undergone by this storage layer 16. If the current is continuous, the magnetisation of the soft layer does not stop turning which can generate a maintained oscillation of the resistance of the magnetoresistive element. This phenomenon is interesting in itself and makes it possible to envisage the production of frequency tunable RF oscillators as demonstrated for example in the publication “Spintorque oscillator using a perpendicular polarizer and a planar free layer” (D. Houssameddine et al., Nature Materials 6, 447 (2007)).
Under the effect of the spin transfer exerted by the perpendicular polariser 20 on the storage layer 16, the magnetisation describes a quasi-circular trajectory in a plane parallel to the plane of the storage layer as illustrated in FIG. 4 which shows this rotation. A cube corner Oxyz (represented in FIGS. 3 and 4) makes it possible to mark out the different directions, the Oz axis being perpendicular to the plane of the layers.
Nevertheless, for a memory application, it is not sought to generate a maintained magnetisation movement in a storage layer but to switch the magnetisation of the storage layer between two stable positions of magnetisation corresponding to two different levels of electrical resistance.
By controlling the duration of the precessional movement generated by the perpendicular polariser to more or less a half-period, it is possible to use the precessional movement to switch the magnetisation of the planar magnetisation layer 16 between two opposite directions. The advantage of this approach is that the switching is in principle very rapid (of the order of 0.3 ns) and not very sensitive to stochastic fluctuations. In fact, the precessional frequencies in these devices are typically in the range of 3 GHz to 10 GHz which corresponds to magnetisation rotation periods of 300 to 100 ps and thus to times of switching the state of the memory cell of 150 to 50 ps (the time to make a half turn being half the time to make a complete turn). Another advantage of this approach is the fact that the writing current pulses may be all of same polarity. Each pulse will trigger a rotation in the direct trigonometric sense (or inverse if the polarity of the pulses is reversed) but the magnetisation may be switched by 180° jumps) while still turning in the same sense.
Nevertheless, such a configuration has at least two drawbacks.
Firstly, it makes it necessary to read the magnetic state of the storage layer before writing. In fact, if the final state that it is wished to write corresponds to the initial state, no current pulse must be sent. It suffices to leave the memory in its current state. On the other hand, if the reading reveals that the memory is not in the state that it is wished to write, it is then necessary to send a current pulse to switch it to the opposite state. The problem is that this step of reading preceding writing may take several nanoseconds thus even if the magnetic switching is then rapid, the fact of having to read before writing lengthens the writing cycle by several nanoseconds which makes this approach unusable for applications requiring writing durations of the order of the sub-nanosecond.
A second drawback resides in the necessity of having to control the duration of the pulses at the scale of 50 ps to be sure to make a half-precession to the magnetisation of the storage layer. The precessional movement leads to a probability oscillating as a function of the duration of the pulses between 0% and 100% of arriving in the state opposite to the starting state (that is to say finish in the parallel state starting from the antiparallel state and vice-versa to finish in the antiparallel state starting from the parallel state). The precession frequency increases with the intensity of the current passing through the device. This leads to a variation in the probability of switching oscillating as a function of the duration of the current pulses at constant current density amplitude or as a function of the current density at constant duration as illustrated in FIG. 5. It is possible to switch the magnetisation of the storage layer by making it do a 1/2 turn but it could be done in principle by making 3/2 turns, 5/2 turns . . . . But in all cases, it is necessary to control the duration of the current pulse to better than more or less ±1/4 period i.e. of the order of 50 ps. This is feasible at the scale of an individual memory point but much more difficult at the scale of an entire memory chip comprising millions of memory points. In fact, due to the resistances of the current lines which interconnect all of the memory points (the aforementioned bit lines and word lines), their parasite capacitances and inductance, a current pulse propagating along the interconnection lines has a tendency to be attenuated and to deform. The pulse thus does not have the same profile at the level of the first memory point and the final one along a same line or column of memory points which makes this precise control of the duration of the pulses very difficult. FIG. 6 shows the probability of switching the storage layer as a function of the duration of the pulse confirming the diagram of FIG. 5.